gitignore: add covdir
gitignore: add covdir

vimalk78/etcd,mqliang/etcd,coreos/etcd,cockroachdb/etcd,whuwxl/etcd,sebrandon1/etcd,rnaveiras/etcd,glevand/coreos--etcd,rnaveiras/etcd,heyitsanthony/etcd,mqliang/etcd,sebrandon1/etcd,lorneli/etcd,ajityagaty/etcd,akauppi/etcd,akauppi/etcd,fanminshi/etcd,dannysauer/etcd,li-ang/etcd,bluebreezecf/etcd,sinsharat/etcd,mkumatag/etcd,cockroachdb/etcd,fanminshi/etcd,li-ang/etcd,glevand/coreos--etcd,etcd-io/etcd,bdarnell/etcd,xiang90/etcd,mqliang/etcd,gyuho/etcd,heyitsanthony/etcd,etcd-io/etcd,etcd-io/etcd,lorneli/etcd,xiang90/etcd,joshix/etcd,bluebreezecf/etcd,dannysauer/etcd,gyuho/etcd,rnaveiras/etcd,sinsharat/etcd,bdarnell/etcd,ajityagaty/etcd,mkumatag/etcd,mkumatag/etcd,coreos/etcd,vimalk78/etcd,ajityagaty/etcd,whuwxl/etcd,joshix/etcd